Plasma guns are utilized for such purposes as thermal spraying which involves the heat softening of a heat fusible material, such as a metal or ceramic, and propelling the softened material in particulate form against a surface to be coated. The heated particles strike the surface and bond thereto. The heat fusible material is typically supplied to the plasma spray gun in the form of powder that is generally below 100 mesh U.S. standard screen size to about 5 microns.
In typical plasma systems an electric arc is created between a water cooled nozzle (anode) and a centrally located cathode. An inert gas passes through the electric arc and is excited thereby to temperatures of up to 15,000 degrees Centigrade. The plasma of at least partially ionized gas issuing from the nozzle resembles an open oxy-acetylene flame.
U.S. Pat. No. 2,960,594 (Thorpe) discloses a basic type of plasma gun. FIG. 1 thereof shows a rod shaped cathode 28 and an anode nozzle 32. The cathode is located coaxially in spaced relationship with the anode nozzle operable to maintain a plasma generating arc between the cathode tip and the anode nozzle. Plasma-forming gas is introduced into an annular space 40 (Thorpe, FIG. 1) surrounding the cathode. This basic structure (without the adjustable cathode or interelectrode segments discussed below) is the type used commercially for such applications as plasma spraying.
Thorpe also depicts in FIG. 1 thereof the mounting of the cathode onto an electrode holder 3 which is threaded into the body of the gun so as to provide adjustment of the position of the cathode. As indicated at column 6, lines 17-24, initial striking of the arc is achieved by screwing the electrode body toward the nozzle and retracting it. An alternative method taught for starting the arc is by providing a high frequency source of current. After the arc is struck the same may be "suitably adjusted" by screwing electrode holder 3. It is also indicated that the tip of the electrode may be positioned at a distance away from the entrance of the nozzle. (Column 6, lines 64-66.) However, the "distance" is limited to relatively small variations, and there is no teaching or suggestion in Thorpe of what position of the cathode is suitable or how to determine such a position.
U.S. Pat. No. 3,627,965 (Zweig) similarly shows a plasma gun with a threaded cathode holder (FIG. 4) and suggests it may be used to alter the arcing gap. Zweig gives no further enlightenment as to the use of the threaded holder.
U.S. Pat. No. 3,242,305 (Kane et al.) discloses a retract starting torch in which starting of the arc is accomplished by a spring urging an electrode against the nozzle. Retraction to a fixed operating position is effected by the fluid pressure of the cooling water.
Zweig also teaches feeding powder inside the gun for spraying. It is well known in the art that such internal feed results in buildup of melted powder inside the nozzle bore. Therefore the conventional powder feeding method which avoids buildup is accomplished by feeding the powder into the flame near or outside the nozzle exit as illustrated in U.S. Pat. Nos. 3,145,287 (Siebein et al.) and 4,445,021 (Irons et al.). This location results in reduced uniformity and effectiveness in heating the powder.
A plurality of electrically isolated interelectrode segments is disclosed in U.S. Pat, No. 3,953,705 (Painter). With reference to the Painter figures these tubular segments are positioned between a nozzle assembly 8 and a rear, fixed electrode 12 of a tubular type, it being generally desirable to have the rear electrode serve as the anode. (Column 8, lines 47-57.) Starting is achieved by application of 20,000 volts which is further increased until the arc occurs. Thus the plasma gun of Painter is for a generally different mode of operation than that of the Thorpe type of plasma gun which has the nozzle as the anode and operates at up to only about 150 volts (Table III of Thorpe). In the low voltage mode the current is high, i.e. of the order of hundreds of amperes, and factors such as arc length and gas type and gas flow establish the operating arc voltage.
As indicated above and illustrated in the above-mentioned patents, the plasma-forming gas is generally introduced into the vicinity of the upstream electrode. Further gas may be injected at at least one point downstream such as is shown in Painter. Other references which show a construction for injecting a second flow of gas are U.S. Pat. Nos. Re. 25,088 (Ducati et al.) and 4,570,048 (Poole). Each of these references shows a fixed cathode.
Plasma guns generally are capable of operating on an inert gas such as argon or nitrogen as the primary plasma gas. For argon the gas is introduced into the chamber near the cathode through one or more orifices with a tangential component to cause a vortical flow to the plasma. The reason is that, without the vortex, the arc is not carried far enough down the nozzle, resulting in low voltage and low thermal efficiency. On the other hand, radial input is generally selected for nitrogen because a vortex tends to extend the nitrogen arc a long distance down the bore of the nozzle causing difficulty in starting the arc.
However, without a vortex for nitrogen, the voltage and efficiency are low. Therefore, an additive gas such as hydrogen is combined with the nitrogen, having the effect of improving these factors. When argon is used, even with a vortex, the efficiency is undesirably low. Hydrogen is again added where possible, but that gas is often considered undesirable as it may cause brittleness in the sprayed coating. Helium is an alternative additive gas but is expensive and less effective.
In view of the foregoing, an object of the present invention is to provide a novel plasma generating system and a novel method for maintaining a predetermined arc voltage without the use of an additive gas to the plasma-forming gas.
Another object is to provide an improved plasma spray gun including a novel powder injector.
A further object is to provide a novel metod for accurately controlling arc length and voltage at efficient levels in a plasma gun.
These and still further objects will become apparent from the following description read in conjunction with the drawings.